Power-assisted cart retriever with attenuated power output

ABSTRACT

The present invention is a motorized shopping cart retriever that includes a controller that attenuates the power provided to the drive system of the retriever to prevent the retriever from being utilized to move an excessively heavy load of shopping carts. By preventing the retriever and carts from being subjected to overloaded conditions, the operational lives of the retriever and carts are maximized. The retriever is also provided with a burst mode that allows the power provided to the drive system to rapidly increase past the normal attenuated level for a brief period of time in order to reduce the time required to accelerate a retriever loaded with carts to a terminal velocity.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication 60/505,546, which was filed Sep. 23, 2003 and is herebyincorporated in its entirety into the present application.

FIELD OF THE INVENTION

The present invention relates to devices and methods for assisting inthe retrieval of wheeled shopping carts in parking lots. Morespecifically, the present invention relates to devices and methods ofcontrolling power assisted (i.e., motorized) shopping cart retrievers.

BACKGROUND OF THE INVENTION

Retrieving shopping carts in large parking lots is primarily a manualtask. Workers push or pull a column of stacked wheeled shopping cartsfrom various collection areas located throughout the parking lots. Thelonger the column of carts being pushed or pulled, the more difficult itbecomes to control. Thus, workers limit the number of carts when movingcarts manually.

Even with more than one worker, the collecting and guiding of longcolumns of carts is difficult and time consuming. Furthermore, in largeparking lots, workers must push or pull the column of carts greatdistances. The retrieval task can be physically exhausting and mayresult in injury to the workers. Carts also can be damaged as theworkers struggle to push or pull long columns to the store door.

In response to the difficulties associated with manual retrieval ofshopping carts, one known apparatus utilized a motorized cart retrieverfor pulling or pushing a column of shopping carts through the parkinglot in a train-like fashion. In the case where shopping carts arepulled, a rope extends from the motorized retriever and is fastened tothe last cart in the column of carts. Each time a cart is added to thecolumn, the rope must be unfastened from the previous cart, extendedfurther, and then secured to the cart added at the end of the column.The operator then walks alongside the motorized retriever as it pullsthirty-five or more carts.

An improved motorized shopping cart retriever is disclosed in U.S. Pat.No. 5,934,694 to Schugt et al., which issued Aug. 10, 1999, and U.S.Pat. No. 6,220,379 to Schugt et al., which issued Apr. 24, 2001. Both ofthese patents are hereby incorporated by reference in their entiretiesinto the present application.

As indicated in FIG. 1, which is a front perspective view of theretriever 2 disclosed in the referenced patents, the retriever 2 is anelectric-powered vehicle with electronic remote control that allows anoperator to use the retriever for retrieval of wheeled shopping carts.The retriever can be operated in a remote or manual mode. The retriever2 provides a foot pedal, a seat 4 and a steering wheel 6 for manualoperation. The retriever is powered by an electric drive and includesdynamic braking and a parking brake.

An additional method of shopping cart retrieval disclosed in thereferenced patents includes pushing the shopping carts with anon-rideable electric-powered retriever that provides for both manualand remote operation. The retriever is moved to the location of theshopping carts either manually or by remote control. The shopping cartsare attached to the front of the retriever, the unit is operated inremote mode, and the operator guides the shopping carts from the frontof the cart train with the retriever pushing the column of shoppingcarts.

While motorized shopping cart retrievers are advantageous over manualmethods of shopping cart retrieval, the use of motorized retrievers haspresented some new problems. For example, because motorized retrieversare able to push a greater number of shopping carts than can be donemanually, operators of the motorized retrievers often overload theretrievers by running exceedingly long strings of carts that theoperator could not manually move by himself. This overloading can resultin damage to the shopping carts and damage to the retriever due to theexcessive stresses exerted on the carts and retriever by the exceedinglylong string of carts. The great mass and length of such strings of cartsincreases the difficulty in starting, maneuvering and stopping thestrings. This presents a danger to pedestrians and cars in the parkinglot. Also, the great length of such strings of carts can interfere withpedestrian and vehicle traffic in a parking lot.

There is a need in the art for a device that can prevent an operatorfrom overloading a motorized shopping cart retriever. There is also aneed in the art for a method of preventing an operator from overloadinga motorized shopping cart retriever.

SUMMARY OF THE INVENTION

The present invention, in one embodiment, is a motorized shopping cartretriever that includes an electronic controller configured to attenuatethe power output of the retriever's drive system. Attenuating the poweroutput of the drive system provides load control and reduces the riskthat the retriever or its shopping carts will be subjected to overloadconditions.

In one embodiment, the controller has a first mode that limits the powerprovided to the electric motor to a level that is within a range thatwill optimize component life for the retriever and the shopping cartsbeing moved by the retriever. Depending on the embodiment, this may beachieved by attenuating the current or the voltage supplied to theelectric motor.

In one embodiment, the controller also has a second mode that allows thepower provided to the electric motor to exceed the limit of the firstmode. The extra power may be provided for an amount of time that may bevaried according to the circumstances to allow more power to theretriever so as to account for variations in conditions during startupof the of the retriever while loaded.

The present invention, in one embodiment, is a shopping cart retrievercomprising an electric motor, a drive system powered by the electricmotor, a throttle control and an analog or digital controller adapted toprovide power to the electric motor and including a first power limit, asecond power limit, and a burst mode. The throttle control is incommunication with the controller. The first power limit is thecontroller's normal power limit that results when the controllerself-limits its maximum power output through a current or temperaturesensing feature of the controller that exists to prevent damage to thecontroller. The second power limit is selectable and limits thecontrol's maximum power output to a level that is less than that of thefirst power limit. The burst mode allows the control's maximum poweroutput to exceed the second power limit for a limited time before againbecoming subject to the second power limit.

In one embodiment, the second power limit is selected to provide a poweroutput level that optimizes the operational life of a component of theretriever or of a shopping cart. In one embodiment, the second powerlimit is selected to provide a power output level that prevents theretriever from being subjected to an overload condition. In oneembodiment, the burst mode includes a third power limit that isselectable and limits the control's maximum power output to a level thatis less than that of the first power limit and greater than that of thesecond power limit. In one embodiment, the second power limit attenuatescurrent or voltage.

The present invention, in one embodiment, is a method of operating ashopping cart retriever. The method comprises placing a controller inelectrical communication with an electric motor adapted to power a drivesystem of the retriever, placing a throttle control in communicationwith the controller, setting the controller at a first power limit thatis less than a second power limit, and moving at least one shopping cartwith the cart retriever by sending power to the electrical motor fromthe controller at a level no greater than the first power limit althoughthe throttle is set at 100 percent full throttle.

In one embodiment, the second power limit is the controller's normalpower limit that results when the controller self-limits its maximumpower output through a sensing feature of the controller that exists toprevent damage to the controller In one embodiment, the first powerlimit is set to provide a power output level that optimizes theoperational life of a component of the retriever or of a shopping cart.In one embodiment, the first power limit is set to provide a poweroutput level that prevents the retriever from being subjected to anoverload condition. In one embodiment, the first power limit is achievedby attenuating a current or a voltage provided to the electrical motorby the controller.

The present invention, in one embodiment, is a method of operating ashopping cart retriever. The method comprises placing a controller inelectrical communication with an electric motor adapted to power a drivesystem of the retriever, placing a throttle control in communicationwith the controller, setting the controller at a first power limit thatis less than a second power limit, attaching the retriever to at leastone shopping cart; and activating a burst mode to accelerate theretriever and the at least one shopping cart from a complete stop to aterminal velocity.

In one embodiment, the second power limit is the controller's normalpower limit that results when the controller self-limits its maximumpower output through a sensing feature of the controller that exists toprevent damage to the controller. In one embodiment, when activating theburst mode, the power provided to the motor from the controller rapidlyincreases to a burst limit and then rapidly decreases from the burstlimit to reach a level approximate the first limit when the retrieverapproaches the terminal velocity. In one embodiment, the burst limit isless than the second power limit and greater than the first power limit.

In one embodiment, the first power limit is set to provide a poweroutput level that optimizes the operational life of a component of theretriever or of a shopping cart. In one embodiment, the first powerlimit is set to provide a power output level that prevents the retrieverfrom being subjected to an overload condition. In one embodiment, thefirst power limit is achieved by attenuating a current or a voltageprovided to the electrical motor by the controller.

While multiple embodiments are disclosed, still other embodiments of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which shows and describesillustrative embodiments of the invention. As will be realized, theinvention is capable of modifications in various aspects, all withoutdeparting from the spirit and scope of the present invention.Accordingly, the drawings and detailed description are to be regarded asillustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front perspective view of a shopping cart retriever asdisclosed in U.S. Pat. Nos. 5,934,694 and 6,220,379.

FIG. 2 is a sectional side elevation of the retriever of the presentinvention as if taken through section line AA of FIG. 1.

FIG. 3 graphically represents the performance characteristics of theelectric motor when caused to operate in various modes by thecontroller.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, in one embodiment, is a power assisted shoppingcart retriever including an a controller adapted to attenuate the powerdirected to the drive system of the retriever. The present invention isadvantageous because it prevents an operator from operating theretriever in an overloaded condition.

For a detailed description of the present invention, reference is nowmade to FIG. 2, which is a sectional side elevation of the retriever 2of the present invention as if taken through section line AA of FIG. 1.As shown in FIG. 2, in one embodiment, the retriever 2 includes a seat4, a steering wheel 6, an electric motor 8, a drive system 10, acontroller 12, a speed throttle control 14 and wheels 16. The electricmotor 8 powers the drive system 10 and is controlled by a controller 12.The speed throttle control 14 of the retriever 2 communicates with thecontroller 12 whether the speed throttle control 14 is mounted on theretriever 2 or is portable. The drive system 10 includes a geararrangement constructed to transmit rotational shaft power from theelectric motor 8 through a combination of gears and shafts to one ormore of the wheels 16 for the purpose of turning the wheels 16 to movethe retriever 2.

In one embodiment, the controller 12 is an electronic motor controller12 used to control the power provided to the electric motor 8 and, as aresult, the power output of the drive system 10. In one embodiment, thecontroller 12 shall have programmable parameters that control themaximum current available to the electric motor 8. In one embodiment,such a controller 12 will exhibit the motor performance characteristicsgraphically illustrated in FIG. 3.

As shown in FIG. 3, in one embodiment, the controller 12 will have aninternal limit mode, a burst mode, and a selected limit mode that willresult in the following three respective power limits, which are theinternal limit 20, the burst limit 22 and the selected limit 24. Theinternal limit 20, which is represented in FIG. 3 by the uppermosthorizontal dashed line having a light line weight, is the controller'snormal current limitation that results when the controller 12self-limits its maximum power output to the electric motor 8 through thecontroller's temperature or current level sensing to prevent damage tothe controller 12.

In one embodiment, the selected limit 24, which is represented in FIG. 3by the lowermost horizontal dashed line having a light line weight, isin addition to the internal limit 20 and allows the controller 12 tofurther limit the power allotted to the electric motor 8. For example,the controller 12 may be programmed such that the selected limit 24corresponds to power levels that are optimal for the operating life ofthe retriever 2 and shopping cart components. Thus, when the controller12 is operating in selected limit mode and power provided to the motor 8is capped at the selected limit 24, the power output to the drive system10 is maintained within the limits necessary to achieve optimumoperating life for the drive system 10 and the associated components ofthe retriever 2. Furthermore, the retriever 2 is only able to move areduced number of shopping carts as compared to when the controller 12is operating in internal limit mode. In other words, the controller 12when operating in selected limit mode acts as a governor that prevents auser from overloading the retriever 2 with excessively long strings ofshopping carts.

As illustrated in FIG. 3, when the controller 12 is in selected limitmode, the selected limit power draw profile (“SLPDP”) 26 a-26 c, whichis represented in FIG. 3 by a dark solid line, increases rapidly overtime (see segment 26 a) until it reaches the level of the selected limit24 where the SLPDP remains (see segment 26 b) until the retriever 2 andits load (i.e., a string of shopping carts) have reached a terminalvelocity and the SLPDP drops to a level that is below the selected limit24 (see segment 26 c).

As can be understood from FIG. 3, when the controller 12 is in selectedlimit mode, the SLPDP 26 b remains at the selected limit 24 for asubstantial amount of time. This is because the electric motor 8, whenthe controller 12 is in selected limit mode, only provides a limitedamount of power to the drive system 10. Consequently, it takes asubstantially longer amount of time to accelerate the retriever 2 andits load to a terminal velocity when operating at the selected limit 24than it otherwise would were the motor 8 provided with the poweravailable at the internal limit 20.

In one embodiment, to reduce the amount of time needed to accelerate theretriever 2 and its load to terminal velocity, the controller 12 can beplaced in burst mode. As indicated in FIG. 3, when the controller 12 isin burst mode, the burst limit power draw profile (“BLPDP”) 28 a-28 d,which is represented in FIG. 3 by a dark dashed line, may increaserapidly up past the selected limit 24 to peak at the burst limit 22,which is represented in FIG. 3 by the horizontal dashed line having alight line weight and located between the internal limit 20 and theselected limit 24.

As can be understood from FIG. 3, during burst mode, the controller 12,for a certain time period T_(B), allows the electric motor 8 to receiveelectrical current that exceeds the selected limit 24. As a result, overthe burst mode time period T_(B), the electric motor 8 provides anincreased amount of power to the drive system 10, which in turndecreases the time required for the retriever 2 and its load to reach aterminal velocity.

As indicated in FIG. 3, when the retriever 2 and its load areencountering a condition of high frictional resistance (e.g., startingfrom a dead stop) and burst mode is activated, the BLPDP 28 a-28 dincreases rapidly (see segment 28 a) past the selected limit 24 untilpeaking at the burst limit 22 (see segment 28 b). The BLPDP then beginsto rapidly decrease (see segment 28 c) until terminal velocity isachieved at the end of the burst mode time period T_(B) and the BLPDPdrops back below the selected limit 24 (see segment 28 d) as theretriever 2 and its load cruise at the terminal velocity.

The burst mode and its associated spike in power, as indicated in FIG.3, is especially helpful when the retriever 2 is being started from astopped state. This is because the resting friction of the cartretriever 2 and its load is larger than its moving friction, so a “burstof power” (i.e., the burst mode current) allows the controller 12 toprovide current to the electric motor 8 that exceeds the currentavailable in selected limit mode. This allows the retriever 2 and itsload to more quickly reach a terminal velocity. As a result, a smalleramount of time is spent accelerating the retriever 2 and load toterminal speed, which allows the retriever 2 to be maneuvered andoperated more efficiently.

In one embodiment, the controller 12 will provide extra power on startupto allow for greater variations in environmental conditions such assnow, ice, extreme temperatures, etc. that affect the load the cartsplace on the motorized cart retriever 2. In one embodiment, thecontroller 12 is provided with a burst mode feature that may be enabledor disabled. The burst mode feature may also provide the ability toadjust, set or reset the rate at which the burst mode current level willrise.

In one embodiment, the burst mode feature will provide the ability toset the length of the time period T_(B) during which the burst modecurrent will be in effect. The burst mode feature may also provide theability to determine or specify the conditions necessary for the burstmode feature to operate. For example, the burst mode feature may providethe ability to determine or specify a minimum time period between burstmode activations. The burst mode feature may also provide the ability toselect a return to zero controller output current, voltage, or power toindicate that the retriever 2 has been or is stopped. A power-down/offindication from the controller 12 and/or on/off button or key switch mayalso be provided. Also, an indicator may be provided to show that thethrottle is going to zero, thereby indicating that the retriever 2 is orwill stop.

In one embodiment, the controller 12 limits the power output of theelectric motor 8 by limiting the current provided to the electric motor8. In one embodiment, the controller 12 limits the power output of theelectric motor 8 by limiting the voltage provided to the electric motor8.

As is well known in the art, electric power of an electric motor 8 isthe product of electric current multiplied by voltage, and mechanicaloutput power from a drive system 10 is directly related to the productof electric power input to the motor 8 and the ratio of the gears. Inthis way the controller 12 can directly control the mechanical outputpower of the drive system 10 by limiting either the current or voltageprovided to the electric motor 8.

The controller 12 may be constructed in analog or digital form so as toprovide an ability to limit the electric current or voltage and toprovide an ability to set the burst limit 22 and the selected limit 24.Exemplary controllers 12 that can be used to limit current to achievethese desired result are (but not limited to) models CS 1108, CS1125,and CS1126 made by Control Solutions, Inc. of 508 W. 5th Ave.,Naperville, Ill. 60563, or model PMA 90-1220 made by PML Flightlink,Ltd. of Alton, Hampshire GU34ZQW England. Variations of these featurescan be found in other manufacturers' controllers 12.

In one embodiment, the electrical motor 8 is a brush DC series wound orseparately excited motor. In one embodiment, the motor 8 is a brushlessDC motor. In one embodiment, the motor 8 is an AC motor. In each case, acontroller 12 is provided that is compatible with the motor 8 utilized.

As previously mentioned, using the controller 12 to limit the poweroutput of the electrical motor 8 effectively limits the power to pull orpush shopping carts by reducing the maximum torque applied to the drivesystem 10. This limits the total number of shopping carts that theretriever 2 is able to pull or push during retrieval. The power to pushor pull multiple carts is directly related to the product of the numberof carts (and their respective weight) and the power to pull or push anindividual cart. Because the power output of the retriever 2 is limitedand the power to push multiple carts varies directly with the number ofcarts, the retriever will be able to move a number of carts (or a totalweight of carts) up to a maximum, but it may not be able to move greaterthan the maximum number of carts.

The electrical motor 8 can provide power to the drive system 10 at alevel that promotes longer component life if the motor power isregularly controlled to be below levels that would cause failures.Alternately, if the electrical motor 8 regularly provides power to thedrive system 10 at levels close to failure, the life of the drive system10 or other vehicle components may be reduced. Further, if the electricmotor 8 is regularly allowed to provide power to the drive system 10 atlevels that exceed failure levels, then drive system or vehiclecomponents may soon fail. This level of drive power may be termed as anoverload of the vehicle. Component tests may be conducted and/orcomponent specifications may be reviewed to determine the power levelthat is appropriate to optimize retriever 2 and cart component life.Once the appropriate power level is determined, the level may be used toset the selected limit 24. A similar method may be used to determine theappropriate level for the burst limit 22.

By using the controller 12 to limit the power output of the electricalmotor 8, the owner of the retriever 2 is able to supervise/control thetotal number of carts retrieved with the retriever 2 by a user. In otherwords, the owner is able to limit the number of carts that a user maypush/pull with the retriever 2, thereby decreasing the likelihood thatthe retriever 2, carts or other property will be damaged by attemptingto move excessively long strings of shopping carts. Because the selectedlimit 24 and the burst limit 22 may be programmed into the controller12, the supervision is available to the owner even when the owner is notpresent when the retriever 2 is being used by a user.

Unlike the present invention, when a user moves shopping carts with aprior art retriever, the user can run the retriever at a power levelequal to the internal power limit 20 by simply placing the throttlecontrol 14 at 100 percent full throttle. Although many prior artretrievers are capable of being run at 100 percent of the internal powerlimit 20, it is often unwise to do so because such power levels oftenoverload one or more parts of the retriever and/or carts, therebycausing premature part failure.

When a controller 12 has a selected power limit 24 that is less than theinternal power limit 20 (as is the case with the present invention), anda user places the speed throttle control 14 at 100 percent fullthrottle, the controller 12 may only provide power up to the selectedpower limit 24, not up to the internal power limit 20. Thus, theselected power limit 24 acts as a governor to prevent the retriever 2from being subjected to overload conditions.

In one embodiment of the present invention, where the controller 12 alsoincludes a burst mode, when a user places the speed throttle control 14at 100 percent full throttle to start a retriever 2 and a string ofcarts from a dead stop, the power from the controller 12 to the electricmotor 8 does not reach the level of the internal power limit 20 as inprior art retrievers. Instead, the controller 12 allows the power fromthe controller 12 to the electric motor 8 to ramp up to the burst powerlimit 22 and back down to the selected limit 24 over the time periodT_(B) it takes to reach a terminal velocity. This feature allowsincreased power for acceleration, but still keeps the power from thecontroller 12 to the motor 8 from approaching levels where overloadconditions may occur (e.g., the internal level 20 or levels approachingthe internal level 20).

In one embodiment, the time period T_(B) is not only the time requiredto reach terminal velocity, it is also a programmable time duration forthe burst current (i.e., burst limit 22). For example, if the retriever2 is truly overloaded, the burst of current may initially move theshopping carts but the time period T_(B) for which the burst current isavailable will be insufficient to get the carts to terminal velocity.Consequently, the retriever 2 will ultimately be unable to continue tomove the carts and the carts will stop because the time period T_(B)timed out and the current level shifted from the burst limit 22 to thelevel of the selected limit 24. This is advantageous because it againlimits the number of carts that can be moved even though there is aburst of current initially available to get the cart column going.

Although the present invention has been described with reference topreferred embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A shopping cart retriever comprising: an electric motor; a drivesystem powered by the electric motor; a controller adapted to providepower to the electric motor and including a first power limit, a secondpower limit, and a burst mode; and a throttle control in communicationwith the controller, wherein the first power limit is the controller'snormal power limit that results when the controller self-limits itsmaximum power output through a sensing feature of the controller thatexists to prevent damage to the controller, wherein the second powerlimit is selectable and limits the controller's maximum power output toa level that is less than that of the first power limit, and wherein theburst mode allows the control's maximum power output to exceed thesecond power limit for a limited time before again becoming subject tothe second power limit.
 2. The retriever of claim 1, wherein the secondpower limit is selected to provide a power output level that optimizesthe operational life of a component of the retriever or of a shoppingcart.
 3. The retriever of claim 1, wherein the second power limit isselected to provide a power output level that prevents the retrieverfrom being subjected to an overload condition.
 4. The retriever of claim1, wherein the burst mode includes a third power limit that isselectable and limits the control's maximum power output to a level thatis less than that of the first power limit and greater than that of thesecond power limit.
 5. The retriever of claim 1, wherein the secondpower limit attenuates current.
 6. The retriever of claim 1, wherein thesecond power limit attenuates voltage.
 7. The retriever of claim 1,wherein the sensing feature is a temperature sensing feature that sensesa temperature of the controller.
 8. The retriever of claim 1, whereinthe sensing feature is a current level sensing feature that senses acurrent level passing through the controller.
 9. The retriever of claim1, wherein the electric motor is a brush DC motor.
 10. The retriever ofclaim 9, wherein the brush DC motor is a series wound DC motor.
 11. Theretriever of claim 9, wherein the brush DC motor is a separately excitedDC motor.
 12. The retriever of claim 1, wherein the electric motor is abrushless DC motor.
 13. The retriever of claim 1, wherein the electricmotor is an AC motor.
 14. A method of operating a shopping cartretriever, the method comprising: placing a controller in electricalcommunication with an electric motor adapted to power a drive system ofthe retriever; placing a throttle control in communication with thecontroller; setting the controller at a first power limit that is lessthan a second power limit, wherein the second power limit is thecontroller's normal power limit that results when the controllerself-limits its maximum power output through a sensing feature of thecontroller that exists to prevent damage to the controller; attachingthe retriever to at least one shopping cart; and activating a burst modeto accelerate the retriever and the at least one shopping cart from acomplete stop to a terminal velocity, wherein power provided to themotor from the controller rapidly increases to a burst limit and thenrapidly decreases from the burst limit to reach a level approximate thefirst limit when the retriever approaches the terminal velocity, whereinthe burst limit is less than the second power limit and greater than thefirst power limit.
 15. The method of claim 14, wherein the first powerlimit is set to provide a power output level that optimizes theoperational life of a component of the retriever or of a shopping cart.16. The method of claim 14, wherein the first power limit is set toprovide a power output level that prevents the retriever from beingsubjected to an overload condition.
 17. The method of claim 14, whereinthe first power limit is achieved by attenuating a current or a voltageprovided to the electrical motor by the controller.
 18. A shopping cartretriever comprising: an electric motor; a drive system powered by theelectric motor; and a controlling means for controlling power to theelectric motor including a first power-limiting means for limiting powerto the electric motor; a second power-limiting means for limiting powerto the electric motor; a burst means for overriding the secondpower-limiting means.
 19. The retriever of claim 18, wherein the secondpower-limiting means limits a power output level to optimize theoperational life of a component of the retriever or of a shopping cart.20. The retriever of claim 18, wherein the second power-limiting meanslimits a power output level to prevent the retriever from beingsubjected to an overload condition.
 21. The retriever of claim 18,wherein the controlling means includes a third power-limiting means forlimiting the power to the electric motor, wherein the thirdpower-limiting means limits a maximum power output of the controllingmeans to a level that is less than that allowed by the firstpower-limiting means and greater than that allowed by the secondpower-limiting means.